In the field of lighting, incandescent or halogen lamps are being replaced today by LED lamps. The low power consumption and long lifetime make them a very useful alternative to the above mentioned conventional light sources. In addition to the use of such LED lamps in new-designed lighting equipment, a particular need exists to retrofit existing lighting systems with LED lamps and thus to replace the above mentioned common types of lamps.
In the before mentioned retrofit application, it is typically necessary to adapt the LED lamp to the respective lighting system installed to allow proper operation, since a change in the setup or wiring of the respective lighting system to be retrofitted, for example installed in an office building, is not easily possible and would result in substantially increased cost of the retrofit process.
A particular example for the above mentioned retrofit application is the replacement of common halogen type lamps in a low-voltage lighting system. Such lighting systems typically comprise a transformer to provide a voltage of e.g. 12V AC. In particular in such lighting systems, various types of dimmers, such as electronic dimmers are employed, operating on the basis of phase cutting, i.e. an adaptation of the RMS voltage according to the desired dimming or brightness level.
While according phase-cut dimmers allow a dimmed operation of a common type of lamp, a reduction of the RMS voltage supplied to an LED lamp does not allow to efficiently dim the LEDs, because of the exponential voltage behavior of LEDs. Instead, the dim or brightness level of LED lamps is typically set by adjusting the current through the LED, e.g. using a current controllable driver unit.
Depending on the lighting system to be retrofitted, it may be useful to allow a dimmed operation according to the brightness or dim level, set by a user using the installed phase-cut dimmer. When retrofitting such lighting system, the desired dim level thus needs to be “extracted” from the operating voltage to allow an accordingly dimmed operating of the LED lamp.
However, noise is caused in the operating voltage by the switching behavior of the above-mentioned phase-cut dimmer, but also due to the switching operation in typically employed electronic transformers. The noise is typically increased due to the fact that LED lamps use considerably less power than conventional lamps, so that the under-load of the dimmer or, in low-voltage systems the dimmer-transformer combination can cause the output of the dimmer to be slightly unstable.
Accordingly, when using the operating voltage to set the current of the LED in an LED lamp, the comprised noise may cause substantial flicker in the output light, visible to the human eye. In particular in the frequency range of 0.1-100 Hz, sometimes even variations in the light output of <1% can be noticed by the human eye and are thus considered disturbing.
Accordingly, it is an object of the present invention to provide a circuit arrangement for an improved control of the brightness of at least one LED to provide a substantially flicker-free light output.